add Travis compiling status.

Author: Yue-Hsun Lin

README.md

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+Travis CI Build Status
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+[![Build Status](https://travis-ci.org/SafeSlingerProject/SafeSlinger-iOS.svg?branch=master)](https://travis-ci.org/SafeSlingerProject/SafeSlinger-iOS)
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 SafeSlinger iOS Client
 ===================
 The open source SafeSlinger Exchange library is a secure and easy to use method of exchanging public keys or other authentication data, with strong protection from Man-In-The-Middle (MITM) attacks. Our goal is to make exchanging public keys as simple as possible without sacrificing security. Our [research paper](http://sparrow.ece.cmu.edu/group/pub/farb_safeslinger_mobicom2013.pdf), presented at MobiCom '13, provides a technical analysis of SafeSlinger's key exchange properties.